Embodiments of the invention relate to an image sensor and a method for manufacturing the same.
An image sensor is a semiconductor device that converts an optical image to an electric signal. Image sensors are generally classified into two categories: charge coupled device (CCD) image sensors and complementary metal oxide silicon (CMOS) image sensors (CIS).
In related art image sensors, photodiodes are formed in a substrate using ion implantation. As the semiconductor industry moves toward ultra-large scale integration, the sizes of photodiodes are progressively reduced to allow the number of pixels in a chip to be increased without an increase in a chip size. In turn, the area of a light receiving portion of the photodiodes may be reduced, which can result in poorer image quality.
Also, since the stack height of device layers over the photodiodes may not be reduced in proportion to the reduction in the area of the light receiving portion, the number of photons incident to the light receiving portion may also be reduced due to the diffraction of light, which results in a phenomenon called “airy disk”.
Alternative methods intended to overcome this limitation include forming a photodiode using amorphous silicon (Si), or forming readout circuitry in a Si substrate (using a method such as wafer-to-wafer bonding) and forming a photodiode on the readout circuitry (referred to as a “three-dimensional (3D)” image sensor). The photodiode is connected with the readout circuitry through a metal line.
According to the related art, two or more insulating layers are formed over the photodiode so as to protect a surface of the photodiode. Since the protection insulating layers are formed of different materials, light incident on the protection insulating layers can be reflected or absorbed, reducing the sensitivity of the photodiode.
Also, in an image sensor according to the related art, leakage current is generated due to environmental elements such as temperature, which may cause a dark current in the photodiode.
In addition, according to the related art, since the source and drain of a transfer transistor connected to the photodiode are heavily doped with an N-type impurity, a charge sharing phenomenon may be generated. The generation of the charge sharing phenomenon may lower the sensitivity of an output image as well as generate an image error. Furthermore, according to the related art, since photo charges may not be transferred between the photodiode and the readout circuitry as efficiently as may be desired, dark current may be generated, or saturation and sensitivity may be lowered.